1. Field of Endeavor
The teachings in accordance with exemplary embodiments of this disclosure generally relate to an inverter, and more particularly, to an inverter with power cell of dual structure configured to reduce product development cost, manufacturing cost and volume of the product by changing a conventional 6-level cascaded H-bridge multilevel inverter for use even in high input voltage.
2. Background
SMPS (Switching Mode Power Supply) is used as a current stabilization power for various electric/electronic/communication devices by controlling flow of power using a semiconductor switching process. Recently, concomitant with switching function improvement of semiconductors and technical development of integrated circuits, miniaturization of SMPS has been greatly implemented and research for reliability and efficiency improvement is actively progressed.
Each phase of a conventional 6-level cascaded H-bridge multilevel inverter includes a plurality of serially-connected unit cells, where each unit cell has an independent single phase inverter structure, and a high voltage can be obtained using low voltage unit cells, that is, low voltage power semiconductors by connecting a plurality of unit cells in series. An input side to which power of each unit cell is applied is connected to a phase shift transformer, and the phase shift transformer provides an independent power to each unit cell of the cascaded H-bridge multilevel inverter.
In order to rectify a 3-phase AC power, the unit cell having an independent single phase inverter structure includes a rectification unit including a diode and thyristors, a DC link unit configured to smooth the rectified voltage and an inverter unit configured to generate a PWM (Pulse Width Modulation) voltage by switching an IGBT using a PWM signal.
Meanwhile, in order to control the rectification unit and inverter unit in each unit cell, an inverter includes an SMPS and a controller configured to control the SMPS, and the SMPS applied for inverter basically uses a DC power outputted from a DC link unit as a power source, and generates an IGBT gate signal for driving a power switch elements of inverter unit by receiving a PWM signal from the controller, and generates a SCR (Silicon-Controller Rectifier) gate signal for driving the thyristor, whereby the thyristor and the power switch element are respectively driven by receiving the IGBT gate signal and the SCR gate signal. At this time, a conventional cascaded H-bridge multilevel inverter is of 6-level type and includes a total of 18 unit cells. Thus, an SMPS configured to control operation of each unit cell and a controller configured to control the SMPS must also include the same number as that of the unit cells.
Meanwhile, the currently-used unit cell receives, as an input, 635V shifted from a voltage of 6600V inputted into the system through a phase shift transformer, where a DC link voltage is 890V. Recently, demands increase on medium voltage inverter to various input voltages in the medium voltage inverter fields, which means that development of SMPS is required capable of using various DC voltages. The currently developed and used SMPS is a type that uses DC power of 890V to a 635V input based on unit cell, such that the unit cell receiving a 1270V input results in a problem of re-developing an SMPS capable of being used in this voltage level.
Particularly, as the input voltage increases, there arise lots of difficulties involving restrictions on insulation of circuits and ratings of core switching elements to disadvantageously generate increased development costs and decreased reliability of product. Thus, reviews and researches are required for medium voltage inverters capable of catering to market demands and securing reliability for the product.